Dam removal as a management strategy for fisheries recovery: lessons from the Elwha River nearshore and implications for Formosa land- locked salmon

Large dams have been a component of human engineering for over 5,000 years. Today? There are over 57,000 large dams world-wide (Lehner et al., 2011; Zarfl et al., 2015). China holds 19,000 large dams, the largest number in the world. The United States has the second highest number, at around 10,000 (Graf, 1999; Poff and Hart, 2002). As the large dams of the world age, largescale dam removal is becoming a more frequent restoration action (Ho et al., 2017). Watersheds that hold large dams often have important connections to coastal systems. The relationships between large-scale dams, their removal, and the nearshore ecosystem function are critical to understand, but often not considered in ecosystem restoration planning for large scale dam removal. Through a decade’s worth of collaborative field-work along the nearshore of the largest dam removal in the world, the Elwha River (Fig. 1), we define fish use response to large-scale dam removal. The Elwha drift cell is approximately 20 km long and includes a number of land-forms including: lower river, estuary/delta, embayed shorelines, feeder bluffs, and spit (Shaffer et al., 2012; Fig. 2, Table 1). Over the course of dam removals, upwards of 20 million cubic meters of sediment was released, of which 60% was predicted to reach the coastal system within 5 years of dam removal. The Elwha dam removal began in September 2011 and concluded in September 2014. As of 2016, 95% of the sediment predicted to be delivered to coastal systems had arrived (Randle et al., 1996; Foley et al., 2017). Four main shifts occurred in the nearshore as a direct result of dam removals: 1. Large volumes of sediment were delivered to the delta/estuary/shoreline in a short period (Fig. 3). However, sediment deposition along the sediment-starved shoreline continued to be disrupted due to remaining shoreline armoring, disrupting ecosystem restoration (Lee et al., 2018); 2. New delta and estuary habitats were formed very rapidly (Fig. 3). As of September 2015, approximately 2.6x106 m3 of sediment material has resulted in over 35 ha of new delta habitat, and increased the total area of the Elwha delta to over 150 ha (Shaffer et al., 2017a; Fig. 3); 3. Along the delta and lower river, there was an almost immediate shift in original habitats from tidally influenced to non-tidally influenced Dam removal as a management strategy for fisheries recovery: lessons from the Elwha River nearshore and implications for Formosa landlocked salmon